A capacitive microphone typically comprises a diaphragm, consisting of an electrode attached to a flexible support member, separated by an airgap from a parallel backplate consisting of an electrode attached to a rigid support member. The backplate has one or more acoustic holes that allow air to flow into and out of the airgap. Acoustic pressure incident on the diaphragm causes it to deflect, thereby changing the capacitance of the parallel-plate structure. The change in capacitance is processed by other electronics to provide a corresponding electrical signal.
Although metals have the appropriate electrical properties for the diaphragm and backplate, their chemical fragility makes them vulnerable to the aggressive thermal and chemical environment of device manufacture and also to corrosive attack during the lifetime of the device. For these reasons layers of a more chemically robust dielectric material with desired mechanical properties are typically used as support members FOR the electrodes. Good mechanical sensitivity for operation at low bias voltages is promoted by a thin, large-area, low-stress diaphragm.
When such layered diaphragms and backplates are used, the most desirable arrangement for good electrical sensitivity is to locate the electrodes opposite one another on the airgap sides, rather than on the exterior sides, of the dielectric layers, because dielectric material between the electrodes adds a fixed component in series with the variable airgap capacitance. With the electrodes on the outer surfaces of dielectric layers that are thick enough to provide a series capacitance comparable to the airgap capacitance, the device sensitivity can be decreased due to reduced electric field in the airgap and reduced device capacitance. Also, the location of the electrodes inside the airgap affords them protection from chemical attack during fabrication and use of the device.
Various methods and materials have been used for fabricating these microphones. Organic films have been used for the diaphragm. These are positioned manually on a stamped metal piece or a silicon wafer containing a backplate and then fixed in place by a polymer spray. An alternative approach is to mount the diaphragm on a separate support wafer that is then joined to a backplate water. The use of such films, however, is less than ideal because temperature and humidity effects on the film result in drift in long-term microphone performance.
This problem has been addressed by making solid state microphones using semiconductor techniques. The well-established technology of bulk silicon micromachining, in which a silicon substrate is patterned by etching to from electromechanical structures, has been applied to manufacture of these devices. Typically, the diaphragm and backplate are fashioned on separate silicon wafers which are then bonded together. Silicon nitride is commonly used for the dielectric layers because it has low chemical activity and its film stress can be tailored by adjusting process parameters of the film deposition. After bonding, the pair of waters is diced into individual devices. Alternatively, the two components can be joined after the wafers have been diced into individual device components. In either case, these techniques require some sort of assembly procedure to obtain a complete microphone. This assembly requirement imposes a practical limit on sensor miniaturization. The critical alignment steps are labor-intensive with low yield and give rise to lack of consistency in performance across devices. Also, the bonding processes generally involve high temperatures and may adversely alter the properties of the microphone constituents or integrated circuitry.
To obviate these difficulties, the production of microphones by a single-wafer process using surface micromachining, in which layers deposited onto a silicon substrate are patterned by etching, has been proposed. (See, e.g., Hijab and Muller, "Micromechanical Thin-Film Cavity Structures for Low-Pressure and Acoustic Transducer Applications," in Digest of Technical Papers, Transducers '85, Philadelphia, Pa., pp. 178-81 [1985].) Microphone manufacture according to this process is based on the sacrificial layer etch-release technique. Successive layers are deposited onto a silicon substrate to from a structure including a layer of sacrificial material placed between a backplate and diaphragm. Access holes in the backplate allow introduction of an etchant which makes a cavity in, or releases, the sacrificial material, thereby forming the airgap between the electrodes. The remaining sacrificial material around the cavity fixes the quiescent distance between the diaphragm and backplate. The access holes then act as acoustic holes during the normal operation of the microphone. This approach is compatible with conventional semiconductor processing techniques and is more amenable to monolithic integration of sensor and electronics than are techniques requiring mechanical assembly.
Several issues must be considered in the selection of etchant and sacrificial material for this process. The etchant should act isotropically to underetch the backplate and completely remove sacrificial material to form the cavity without detriment to the diaphragm or backplate. In the case of a composite diaphragm or backplate, the inactivity of the etchant with respect to the material abutting the sacrificial layer is especially important. The ideal sacrificial material would be robust to high-temperature semiconductor processing methods and also depositable without damage to existing layers. Both the deposition and etching of the sacrificial material should be easy to incorporate into conventional semiconductor processing lines. A system with this combination of attributes has not been available.
Examples from the prior art illustrate the difficulties in achieving this ideal. One demonstrated technique is the use of sacrificial aluminum and wet acidic etchant with diaphragm and backplate of silicon nitride support members with aluminum electrodes. (See, e.g., Scheeper et al., Journal of Microelectromechanical Systems, 1, 147-54 [1992].) A typical etchant composition includes a mixture of aqueous acids such as nitric, acetic and phosphoric. Such wet etchants are isotropic with high selectivity between aluminum and silicon nitride. However, capillary forces exerted by the etchant residue cause the diaphragm and backplate to stick together, so the etch must be followed by an elaborate freeze-drying procedure to adequately remove the residue. The freeze-drying is time-consuming and requires a high-vacuum apparatus. Also, because the sacrificial and electrode materials are the same, the electrodes are vulnerable to attack by the etchant and must be nonoptimally located on the sides of the dielectric support layers away from the airgap. Furthermore, the metal lines of the chip circuitry themselves are subject to attack by the acids.
The use of sacrificial silicon and aqueous tetramethylammonium hydroxide etchant with diaphragm and backplate having silicon nitride support members with aluminum electrodes has been proposed, although not demonstrated, by van der Donk (A Silicon Condenser Microphone: Modeling and Electronic Circuitry, doctoral dissertation, Universiteit Twente, [1992]). The description specifies that etching be followed by an aqueous methanol rinse and freeze-drying. Although the sacrificial layer and electrodes are of different materials in this case, the etchant is not sufficiently selective between aluminum and silicon to provide any protection to the electrodes or circuitry.
Use of sacrificial photoresist and either a wet etchant or a dry oxygen-plasma etchant with a electroplated monolithic copper backplate was reported by Bergqvist et al. (Journal of Microelectromechanical Systems, 3, 69 [1992]). Isotropic removal of photoresist by an oxygen-plasma is a well established technique. The dry etchant's selectivity of photoresist over the electrode metal is sufficient to allow location of the backplate electrode inside the air gap; however, the temperature limitations of the photoresist material would not allow its use in conjunction with any higher temperature process-such as chemical vapor deposition ("CCVD")--after the deposit of the sacrificial material. This proscription severely limits process design. Furthermore, although the dry-etch option obviated the need for freeze-drying, in this case it left the backplate severely buckled; this problem was only solved by an adaptation of the wet-etch process.
Accordingly, it is apparent that the prior art has not given rise to an etchant/sacrificial material combination that takes full advantage of the potential of the sacrificial etch approach to microphone fabrication.